I don’t see it as a Rule, exactly, that one is Not Allowed to wear open-toed shoes in winter. You should be guided by the weather, and not by the calendar. But it usually amounts to the same thing:
Since we are Not Allowed to wear stockings with open-toed shoes, if it’s too cold to forgo stockings, it’s too cold for open-toed shoes.
Not Exceptions:
So-called “sandal-foot” stockings: Sorry, they exist purely to trick you into buying more stockings than you need, especially since these will run more easily than your proper reinforced-toe stockings.
Open-toed stockings: Ha! Now I’m trying to trick you. If you are seriously considering stooping to this level just to get a jump on wearing your spring shoes, you need to stop and reexamine your priorities.
Shivering in bare legs: Nothing looks more tasteless than a woman who ought to be wearing stockings but isn’t. Again, no shoes are that cute—just wait until it’s warmer!
Opaque tights: I guess if you are “making a fashion statement,” and the stockings are truly opaque, and completely matte… But it still would just look like you’re trying to cheat.
Patience—it’s one of the pillars of Style.